1. Field of the Invention
The present invention relates to a toner-fixing heater device and, more particularly to a heater roller of the type in which a resistor heater body attached to the inner circumferential surface of a roller body is energized to heat toner deposited on a printing medium for fixing.
2. Description of Related Art
In conventional image printing apparatus such as laser printers and photo copiers, a printing mechanism and a fixing mechanism are provided for depositing toner on a printing medium based on an image signal and for fixing the deposited toner to the printing medium, respectively. The fixing mechanism includes generally a toner-fixing heater roller and a presser roller pressed to the toner-fixing heater roller to heat and fix the deposited toner to the printing medium while feeding the printing medium therebetween.
The toner-fixing heater roller is provided on its roller body with a resistor heater body which is energized for heating up to a temperature (e.g., 140.degree. C.) required for the toner fixing in a short period of time.
JP-A 62-279378 discloses one type of toner-fixing heater roller in which an insulator film layer is formed on the inner circumferential surface of a cylindrical roller body and a resistor heater body layer is formed on the inner circumferential surface of the insulator film layer. Inside the roller body, a pair of ring-shaped electrically conductive members are contacted to the resistor heater body layer to supply the electric power to the resistor heater body for heating. This type of the toner-fixing heater roller, however, requires a very complicated work for forming the insulator film layer on the roller body and then forming the resistor heater body layer on the insulator film layer. In addition, it requires a very complicated structure of the ring-shaped electrically conductive member which is contacted to the resistor heater body layer.
JP-A 8-194401 (U.S. Pat. No. 5,575,942) discloses another type of toner-fixing heater roller in which a resistor heater body is constructed by a strip heater meandering in a predetermined pattern and a pair of generally ring-shaped heater electrodes formed integrally on both sides of the strip heater. The resistor heater body is fixed to an insulator film layer, thereby forming a resistor heater sheet. This resistor heater sheet is attached to the inner circumferential surface of a roller body so that the resistor heater body heats when the electric power is supplied to the heater electrodes by contacting brush electrodes.
In this type of the toner-fixing heater roller, a discontinuous part is formed at one circumferential location where a pair of circumferential ends of each heater electrode reside, so that the resistor heater sheet is fixed to the inner circumferential surface of the roller body. This discontinuous part is formed to extend in an axial direction perpendicular to the circumferential direction of the roller body, i.e., at the right angle relative to the sliding contact direction of the brush electrode. The heater electrode receives a greater contact force when the brush electrode slidingly moves from the discontinuous part to its circumferential end. Thus, the discontinuous part impedes smooth sliding of the brush electrode and causes wear in the heater electrode. Further, it causes electric noises when the brush electrode fails to contact the heater electrode temporarily.